Influence of EP and AW additives on the tribological behaviour of hard low friction coatings

The aim of the present work was to investigate the influence of conventional extreme-pressure (EP) and anti-wear (AW) additives on the tribological performance of hard low-friction coatings in the boundary lubrication regime. Tests were performed in a load-scanning test rig, where DLC coated steel bars were loaded against coated and uncoated bars, while the steel/steel combination was used as a reference. This test configuration involves two crossed bars that are forced to slide reciprocally against each other under a constant speed, allowing the normal load to gradually increase during the forward stroke and to correspondingly decrease during the reverse stroke. Load-scanning tests were performed under boundary lubrication conditions using four lubricants; pure poly-alpha-olefin oil (PAO), PAO mixed with commercial sulfur-based EP additive or ZDP-based AW additives, and fully formulated gearbox oil. Results of this investigation show that also under boundary lubrication conditions introduction of hard low-friction coatings and usage of additivated oils significantly improve the tribological performance. Furthermore, the coatings were found to have a great influence on the running-in process.